This invention relates to a monolithic ceramic capacitor having buried electrodes, at least one of which floats and overlaps two main buried electrodes that lie in a common plane, each of the main electrodes extending to separate body faces of the ceramic capacitor body to provide electrical access thereto.
There has long been a need for a stable, precise-value, low-capacity monolithic ceramic capacitor. Low-dielectric-constant (K) ceramic materials usually provide the needed stability. Such materials have a K of less than about 200. The needed small values, e.g. less than about 200 pf, are easily obtained in an inexpensive monolithic ceramic capacitor having a convenient chip geometry (right-parallelepiped) and small size, e.g. largest dimension ranging from 0.20 to 0.040 inch (5 to 1 mm). It has heretofore not been possible to manufacture such capacitors to tighter capacitance tolerances than about .+-.20%, i.e. the distribution of capacitance values typically has 2.sigma. points at about .+-.20% of the nominal capacitance value. There is much demand for monolithic ceramic capacitors guaranteeing a particular low capacitance value .+-.2% or even .+-.1%. Only capacitors that are found by testing to meet these limits can be supplied to such purchase specifications.
With rare exception, monolithic chip capacitors include a first group of mutually-parallel buried electrodes extending to one body face for electrical access, and a second group of mutually-parallel, buried electrodes interdigitated with the first group and extending to another body face. In most such capacitors, the adjacent electrodes are of the same dimensions and are meant to be registered one directly over the other. Any misregistration of those adjacent electrodes in manufacturing causes a commensurate deviation from the nominal capacity value of the finished capacitor, i.e. an elecrodes misregistration of 5%, which is substantially more than is usually allowed, leads to a capacity change of 5%. It would therefore be possible to purposefully offset the registration between adjacent electrode patterns to adjust the capacity value to a desired value, but that would be of small practical value for capacitors of small capacity value since the degree to which registration can be maintained is inversely proportional to the overlap areas of adjacent electrodes.
This precision-degrading factor can be eliminated by using a floating electrode and adjusting the relative geometries of adjacent electrodes as described in my patent, U.S. Pat. No. 3,896,354 issued July 22, 1975 and assigned to the same assignee as is the present invention. In one example, a floating electrode in one plane has a fixed overlap with each of two other electrodes in an adjacent plane that extend to opposite body faces. Relative movement between the electrode patterns in each of the two planes has no effect on the capacity between them. This technique, however, does not solve the problem of how to manufacture close tolerance capacitors to a predetermined nominal capacity value.
That has been accomplished in the past by adjusting the capacity value after the capacitor had been through the sintering and termination steps. In one case a pattern of very small electrode strips has been interposed between at least one pair of adjacent main electrodes that extend to a first terminal face of the body. The small interposed strips extend to a third face and are connected as needed to the main electrodes extending to the second face to adjust the capacity value upward. In another case the finished capacity body is abraded to dig a cavity in a central portion of the body to penetrate and remove at least a portion of the outermost buried electrode or electrodes to effect a downward capacity adjustment. This method is described by Hatch in U.S. Pat. No. 3,456,170, issued July 15, 1969 and assigned to the same assignee as is the present invention. However, making adjustments in capacity for very small chips in accordance with these methods is just not practical, and even for larger chips it would be advantageous to make the capacitors to the desired nominal capacity value in the first place and avoid the extra steps and costs involved in such methods of capacity adjustment.
It is therefore an object of this invention to provide a monolithic ceramic capacitor that can be manufactured to tight capacitance tolerances.
It is a further object of this invention to provide a method by which such capacitors can be made.